The present invention relates to the treatment of gas streams, particularly gas streams containing non-condensible gases (NCG""s) including total reduced sulfur (TRs) gases.
Non-condensible gases (NCG), which include reduced sulfur compounds, are various gaseous components in tail gas streams arising from various industrial sources. In many industries, including the chemical and pulp and paper industries, the emission of these gas streams containing reduced sulfur compounds presents a major challenge related to the elimination of the odor associated with the compounds. The odors are foul and people can smell their presence at parts per billion levels. Typical reduced sulfur compounds are hydrogen sulfide (H2S), dimethyl sulfide (DMS) and any of its homologs, dimethyl disulfide (DMDS) and any of its homologs, mercaptans including methyl mercaptans and its homologues, and various other sulfur-containing compounds. The homologs of methyl mercaptan, DMS and DDMS include the corresponding ethyl-, propyl- or butyl-organic sulfur compounds.
While there are a number of methods available to remove H2S from gas streams, certain mercaptans and the methyl sulfides are fairly stable and exhibit low solubility in aqueous solutions, especially DMDS, and resist removal by conventional H2S-removal processes.
Canadian Patent No. 643,349 describes the removal of sulfur compounds in a pulp mill environment using spent chlorination effluent from a bleach plant in a pulp mill followed by contact of the heated gas stream with alkali. The spent chlorination effluent contains hypochlorite for the purpose of oxidation of sulfur compounds.
U.S. Pat. No. 4,505,776 describes a procedure in which hydrogen sulfide first is absorbed from flue gas in pulp mill black liquor (spent pulping liquor) and the resulting solution then is oxidized. The contaminated flue gas is initially cooled to near its dew point and scrubbed with water to remove particulates.
For years, non-condensible gases (NCG) in Kraft mills which contain such reduced sulfur compounds, were typically routed to the boilers and lime kilns for treatment. On incineration, the sulfur content of the NCG is convened to SO2, which must then be removed by scrubbing. Lime kilns are not always in operation; in this case the gases would be vented to a stack or routed to a power boiler for treatment, again producing SO2 which must itself be treated.
Since mid-1997 in the U.S., all NCG-containing gases must be treated by a fully committed process to below about 5 ppm before release. Kraft mills have many sources of such gases, including emissions from chemical recovery boilers, digesters, brown stock washers, effluent treatment systems, etc.
Incineration of NCG""s is expensive in that it requires an incinerator (an expensive energy intensive approach) and an SO2 scrubber.
We have conducted pilot studies at a number of Kraft mills over the past twenty years. Such tests were conducted on recovery boiler emissions at four different mills. In early tests (1975) conducted at a MacMillan Bloedel mill at Portage-du-Fort, Quebec, the task was to recover the salt cake not captured by The electrostatic precipitator. Excellent results were obtained. However, it was noted that the TRS had been significantly removed in the scrubber. The second such test (1987) was conducted at a Boise Cascade mill located in Fort Francis, Ontario with similar results. The next thee tests were conducted at a MacMillan Bloedel mill located at Powell River, B.C., the first to demonstrate salt cake removal and the next two to demonstrate removal of TRS compounds. A subsequent test was performed at a Cascades mill in East Angus, Quebec.
Using various chemicals in the scrubbing liquors, it was found that high salt cake recovery could be coupled with removal of TRS primarily H2S to undetectable levels. It was subsequently found that the chemistry used at the Powell River and East Angus mills could not adequately destroy DMDS.
In U.S. Pat. No. 5,192,517. assigned to the assignee hereof and the disclosure of which is incorporated herein by reference, there are described gas/liquid contact systems for the removal of solute gases containing sulfur dioxide by contact of the gas stream in a conduit with atomized sprays of aqueous alkaline medium delivered by non-plugging dual-fluid nozzles which form droplets sized about 5 to about 100 microns.
Utilization of various specific chemistries and pHs coupled with variables inherent to the scrubbers described in U.S. Pat. No. 5,192,517, however, has led to the development of a complete solution to the problem, as provided herein. During the tests at a James River Corporation mill in Alabama, the mill used gas chromatography on inlet and outlet samples to and from the scrubber. It was firmly established during these tests that all reduced sulfur compounds were reduced to below detection limits.
In accordance with the present invention, there is provided a novel procedure for the elimination of NCG-containing gases and odours associated with them. Such procedure involves, optionally, prescrubbing the odorous gas stream by intimate gas-liquid contact to cool the gas to its adiabatic dewpoint, if necessary, while simultaneously removing particulate matter present in the gas steam, scrubbing the prescrubbed gas with an oxidizing liquid which is an aqueous solution of chlorine dioxide by intimate gas/liquid and gas/gas contact in one or more contact stages to oxidize reduced sulfur compounds and any other oxidizable compounds present in the prescrubbed gas stream, such contacting being erected by finely atomizing the aqueous solution of chlorine dioxide into the gas stream in each of the contact stages to form a spray of droplet size distribution of about 5 to about 250 microns Sauter Mean Diameter (SMD), and then passing the treated gases through an entrainment separator, for example, a mist eliminator, to remove entrained liquid droplets from the gas stream prior to discharge to a stack. The other oxidizable compounds which may be present in the treated gas stream include methanol or other alcohol, such as ethanol, propanol or butanol. The latter materials also may be separated in an additional operation.
The use of an aqueous solution of chlorine dioxide as the oxidizing liquid results in a more efficient removal of the sulk gases, particularly dimethyl sulfide and dimethyl disulfide, which are of limited solubility in aqueous media, than other oxidizing material, such as the sodium hypochlorite described in the prior an referred to above.
An aqueous solution of chlorine dioxide can be atomized utilizing very low energy levels and provides both oxidation in solution in the liquid droplets for gases absorbed therein and gaseous phase oxidation, resulting from partial gassing off of chlorine dioxide as the atomized solution contacts the containment-laden, high temperature gas stream, for gases remaining in the gaseous phase. This combined oxidation results in effective high efficiency scrubbing of reduced sulfa gases, easily achieving a regulatory requirement of residual values of reduced sulfur compounds below 5 ppm.
If necessary, the gases leaving a first oxidation stage entrainment separator may be contacted with a neutralizing and reducing solution by intimate gas-liquid contact in one or more contact stages to remove acidic compounds, such as H2SO4 and/or HCl, formed in the oxidizing stage reaction while reducing any residual chlorine dioxide remaining from the first oxidation stage present in the gas stream. The neutralizing and reducing solution may include an aqueous caustic solution alone or in combination with one or more reducing agents. Such intimate gas-liquid contact may be effected by finely atomizing the aqueous neutralizing and reducing solution into the gas stream in each of the contact stages to form a spray of droplet size distribution of about 5 to about 100 SMD.
White liquor (Kraft mill pulping liquor), which may be considered to be a very basic solution of sodium hydroxide and sodium sulfide, is a particularly useful material for this purpose, since the high pH component effectively neutralizes acidic gases in the gas stream while the sulfide portion effectively reduces any chlorine dioxide oxidant remaining in the gas stream as it is oxidized to sulfate. White liquor usually is used in a Kraft pulp mill for pulping wood chips to release the cellulose fibers from the wood chips and, when the process is applied to NCGs in a Kraft pulp mill, is a readily-available material.
Such treated gases then may pass through a second entrainment separator stage using an appropriate entrainment separator prior to discharge from the system through an exhaust stack.